Splicing, Exporting RNA, & Reading the Genetic Code
Learning Objectives In-Depth Explanations
Explain when and where splicing occurs and why correct regulation of splicing is important for cell functions:
Splicing occurs in the nucleus of eukaryotic cells after a gene has been transcribed into pre-mRNA. It involves removing introns (non-coding regions) and joining exons (coding regions) to form mature mRNA. Correct regulation is vital because errors in splicing can lead to non-functional proteins or proteins with altered functions, which can cause diseases.
Describe the steps involved in mRNA splicing, including how splice sites are selected:
mRNA splicing involves several steps: (1) the spliceosome complex recognizes and binds to specific sequences at the 5' and 3' ends of introns, as well as the branch point within the intron; (2) the intron is excised in the form of a lariat structure; and (3) the exons are joined together. Splice sites are selected based on consensus sequences that are typically found at the exon-intron boundaries. snRNAs within the spliceosome base-pair with these sequences to ensure accurate splicing.
Explain how errors in splicing could occur and impact the health of cells/organisms:
Errors in splicing can occur due to mutations in the splice site sequences or in the snRNAs. These errors can lead to:
Exon skipping: An exon is mistakenly removed along with the introns.
Intron retention: An intron is not removed and remains in the mature mRNA.
Cryptic splice site selection: The spliceosome uses a site that resembles a normal splice site but is not typically used.
These errors can result in frameshifts, premature stop codons, or altered protein sequences, leading to non-functional or abnormally functioning proteins, which can cause various diseases such as beta-thalassemia.
If given information about a gene sequence (i.e. introns, exons, splice sites, cryptic splice sites), predict how splicing could be affected by a mutation and what changes this would lead to in the amino acid composition of the resulting protein:
Given a gene sequence, one can analyze the potential impact of mutations on splicing. Mutations at splice sites may abolish splicing at that site, leading to exon skipping or intron retention. Mutations creating new splice sites (cryptic splice sites) can lead to inclusion of additional sequences in the mRNA. These changes will alter the mRNA sequence, potentially leading to frameshifts or altered amino acid sequences in the resulting protein. Using a codon table, you can predict how these changes in mRNA sequence will affect the amino acid sequence.
Explain how mRNAs are selected for export from the nucleus and what happens to the RNA left behind:
mRNAs are selected for export from the nucleus based on their association with specific proteins that signal their readiness for translation. These proteins, including the exon junction complex (EJC), bind to the mRNA after successful splicing. Nuclear transport receptors recognize these proteins and facilitate the export of the mRNA through nuclear pore complexes. RNA that is not properly processed, such as introns or incorrectly spliced products, are retained in the nucleus and degraded by the RNA exosome.
Explain what the rRNA S values refer to:
The "S" in rRNA S values (e.g., 28S, 18S, 5S) stands for Svedberg units, which are a measure of the sedimentation rate of a molecule during centrifugation. The S value depends on both the molecule's mass and shape. Larger S values indicate larger and more compact molecules.
Explain why there are many repetitive copies of rRNA and tRNA genes in the genome:
rRNA and tRNA are essential for protein synthesis, and cells require large quantities of these molecules to maintain high rates of translation. Having multiple copies of rRNA and tRNA genes ensures that cells can produce sufficient amounts of these RNAs to meet their translational demands.
Explain the functions of rRNA, tRNA, snoRNA and how/where they are transcribed and processed:
rRNA (ribosomal RNA): Forms the structural and catalytic core of ribosomes, the protein synthesis machinery. rRNA is transcribed from rDNA in the nucleolus by RNA polymerase I (except for 5S rRNA, which is transcribed by RNA polymerase III outside the nucleolus). Pre-rRNA is processed through cleavage and modification (methylation and pseudouridylation) guided by snoRNAs.
tRNA (transfer RNA): Adapters that match codons in mRNA to specific amino acids during translation. tRNAs are transcribed by RNA polymerase III. Pre-tRNAs are processed by trimming, splicing, and base modifications.
snoRNA (small nucleolar RNA): Guide chemical modifications (methylation, pseudouridylation) of rRNA. snoRNAs are transcribed mainly from introns of protein-coding genes by RNA polymerase II in the nucleolus.
Describe how the genetic code is read, including what is meant by “frame” and “non-overlapping”:
The genetic code is read in a sequential manner, where each codon (a sequence of three nucleotides) specifies a particular amino acid or a stop signal.
Frame: The reading frame refers to the starting point for reading the genetic code. Since the code is read in triplets, there are three possible reading frames for any given mRNA sequence. Only one of these frames is typically correct and encodes the functional protein.
Non-overlapping: The code is non-overlapping, meaning that each nucleotide is part of only one codon. The ribosome moves three nucleotides at a time.
Describe the structure of a tRNA and how tRNA contribute to reading the genetic code:
tRNAs have a characteristic cloverleaf secondary structure and an L-shaped tertiary structure. Key features include:
Anticodon loop: Contains a three-nucleotide sequence (anticodon) that base-pairs with a complementary codon on the mRNA.
Acceptor stem: At the 3' end, where a specific amino acid is attached.
tRNAs contribute to reading the genetic code by:
Recognizing specific codons in the mRNA through codon-anticodon pairing.
Delivering the corresponding amino acid to the ribosome for incorporation into the growing polypeptide chain.
Explain how the same amino acid could be encoded by 2 different codons and what is meant by “wobble”:
The genetic code is degenerate, meaning that multiple codons can encode the same amino acid. This is possible because of "wobble" in the third base of the codon.
Wobble: Refers to the flexible base-pairing between the third base of a codon and the corresponding base of the tRNA anticodon. This allows a single tRNA to recognize more than one codon, as long as the first two codon-anticodon base pairs are correct.
From a DNA gene sequence (and codon usage table) predict the encoded sequence of mRNA and amino acids:
Transcribe DNA to mRNA: Convert the DNA sequence to mRNA by replacing T with U.
Identify the Correct Reading Frame: Locate the start codon (AUG) to establish the correct reading frame.
Divide mRNA into Codons: Split the mRNA sequence into consecutive triplets (codons).
Use Codon Table: Use the reference codon usage table to find which amino acid each codon encodes.
Translate: Chain the amino acids together in the sequence dictated by the codons to determine the final protein sequence.
Differentiate between silent mutations, nonsense mutations, frameshift mutations, missense mutations, translocations, and insertions/deletions and predict if/how each could impact polypeptide expression or function:
Silent mutations: Change a nucleotide but do not change the encoded amino acid due to the degeneracy of the genetic code. They have no impact on polypeptide expression or function.
Nonsense mutations: Introduce a premature stop codon, leading to a truncated protein. This typically results in a non-functional protein due to the loss of essential domains or misfolding.
Frameshift mutations: Insertions or deletions of nucleotides that are not multiples of three, causing a shift in the reading frame. This leads to a completely different amino acid sequence downstream of the mutation, often resulting in a non-functional protein due to misfolding or premature stop codons.
Missense mutations: Change a nucleotide and alter the encoded amino acid. The impact on polypeptide function depends on the nature and location of the amino acid change. Some missense mutations may have minimal effects, while others can disrupt protein folding, stability, or activity.
Translocations: Occur when a segment of a chromosome breaks off and attaches to another chromosome. This can disrupt gene expression if it separates a gene from its regulatory sequences or creates a fusion gene.
Insertions/Deletions: Add or remove one or more nucleotides. If not a multiple of three, these cause frameshift mutations.
Describe the functions of aminoacyl-tRNA synthetase and how this contributes to accuracy of translation:
Aminoacyl-tRNA synthetases are enzymes that catalyze the attachment of the correct amino acid to its corresponding tRNA. This process, called tRNA charging, is crucial for ensuring the accuracy of translation. Each aminoacyl-tRNA synthetase is highly specific for a particular amino acid and tRNA.
Accuracy: These enzymes have proofreading mechanisms to ensure that the correct amino acid is attached to the tRNA, minimizing errors in translation.
Compare the # of different aminoacyl-tRNA synthetases to the # of different tRNAs, codons, and amino acids:
There is generally one aminoacyl-tRNA synthetase for each amino acid (20 in most organisms). However, there can be multiple tRNAs for a single amino acid due to codon degeneracy.
Therefore:
# of aminoacyl-tRNA synthetases ≈ # of amino acids (20)
# of tRNAs > # of aminoacyl-tRNA synthetases (often 30-50)
# of codons > # of tRNAs (61 sense codons)
Identify the structure and production site of eukaryotic ribosomes (and components) and where translation occurs:
Eukaryotic ribosomes are composed of two subunits: a large subunit (60S) and a small subunit (40S). Each subunit consists of rRNA molecules and ribosomal proteins.
Production Site: Ribosome biogenesis occurs primarily in the nucleolus. rRNA genes are transcribed, processed, and assembled with ribosomal proteins to form the ribosome subunits.
Translation Location: Translation occurs in the cytoplasm, where the two ribosomal subunits come together on the mRNA molecule to synthesize proteins.
Define “ribozyme” and be able to recognize examples of molecules/complexes that function in this way:
Ribozyme: An RNA molecule that possesses catalytic activity. Ribozymes can catalyze specific biochemical reactions, similar to protein enzymes.
Examples:
Ribosomes: The rRNA within the ribosome catalyzes the formation of peptide bonds during protein synthesis.
Spliceosome: snRNAs catalyze splicing.